Ultra-high-strength and ultra-high-toughness oil casing and manufacturing method thereof

ABSTRACT

There is disclosed an ultrahigh strength and ultrahigh toughness oil casing pipe, comprising the following chemical elements in mass percentages: C: 0.12-0.18%; Si: 0.1-0.4%; Mn: 1.1-1.6%; Cr: 0.1-0.4%; Mo: 0.2-0.5%; Nb: 0.02-0.04%; Ti: 0.02-0.05%; B: 0.0015-0.005%; Al: 0.01-0.05%; Ca: 0.0005-0.005%; N≦0.008%; with the proviso of 0&lt;(Ti—3.4N)≦0.02%, Ti/B≧10; and the balance of Fe and other unavoidable impurities. The strength of the oil casing pipe can be equal to or higher than 150 ksi steel grade, and the zero degree lateral Charpy impact work is not less than 10% of the yield strength of the 150 ksi steel grade.

TECHNICAL FIELD

The disclosure relates to a metallurgical product and a method of making the same, particularly to an oil casing pipe and a method of making the same.

BACKGROUND ART

Nowadays, increasing attention is paid to exploitation of oil and gas resources in deep and ultradeep wells around the world. The resources are buried extremely deeply in West China oil fields which are characterized by complex geological structures. The deepest oil and gas wells are over 8000 m in depth. Consequently, the strength of casing pipes for exploitation in oil and gas wells is required to be increased significantly. It's well known that, as the steel grade and yield strength of a material increases, the hardness of the material increases correspondingly, but the toughness of the material decreases gradually, and the material becomes more sensitive to surface defects. Casing pipes useful for exploitation in deep and ultradeep wells have high requirements for strength and toughness. While high strength is achieved, the toughness index should be maximized to ensure safety in production.

However, steel strength and its toughness and plasticity generally vary in opposite directions. Steel having high strength generally has low plasticity and toughness. Conversely, if steel is desired to have high plasticity and toughness, its strength has to be decreased. Therefore, it's a great challenge to develop a steel material having both high toughness and high strength. Currently, the casing pipes applicable in industry may have a strength up to 170 ksi, but their impact toughness is only 50-80 J. Some related guideline documents point out that the impact toughness of high strength steel for pressure containers must achieve 10% of its yield strength. As such, various domestic large oil fields, such as Tarim Oil Field, also propose the same standard for casing pipes for deep and ultradeep wells. However, the impact toughness property of high strength steel having a strength of 150 ksi (yield strength being 1034 MPa) or above currently available is far below this standard.

A Chinese patent reference (publication number: CN101586450A; publication date: Aug. 27, 2008; title: Petroleum Casing Pipe With High Strength And High Toughness and Preparation Method Thereof) relates to a steel for oil casing pipes, comprising the following chemical element composition (wt %): C: 0.22-0.4%, Si: 0.17-0.35%, Mn: 0.45-0.60%, Cr: 0.95-1.10%, Mo: 0.70-0.80%, Al: 0.015-0.040%, Ni<0.20%, Cu<0.20%, V: 0.070-0.100%, Ca>0.0015%, P<0.010%, S<0.003%, and the balance of iron. This Chinese patent reference also provides a method of preparing this oil casing pipe, comprising the following steps: 1) batching and smelting; 2) continuous casting and rolling; and 3) pipe machining. The steel disclosed by the above Chinese reference has a strength up to 1100 MPa, but its lateral impact toughness is only 90 J, which is a low toughness index.

Another Chinese patent reference (publication number: CN101250671A; publication date: Nov. 25, 2009; title: Petroleum Casing Pipe With High Strength and High Toughness and Preparation Method Thereof) relates to an oil casing pipe and a method of preparing the same, wherein the oil casing pipe comprises the following chemical elements in mass percentages (wt %): C: 0.16-0.28, Si: ≦0.5, Mn: 0.3-1.10, Cr: 0.3-1.10, Mo: 0.60-0.95, Al: 0.015-0.060, wherein acid soluble Als/Al≧0.8, Ni: <0.60, Cu: 0.05-0.25, V: 0.06-0.20, Ca>0.0015, Nb: ≦0.05, Ti: ≦0.05, P<0.010, S<0.002, O: <0.0024, H: <0.0002, N: <0.008, B: 0.0-0.005, and the balance of iron. The lateral impact roughness of the oil casing pipe disclosed by this Chinese patent reference is only 80J, which is also a low toughness index.

A Japanese patent reference (publication number: JPH11-131189A; publication date: May 18, 1999; title: Steel Pipe and Its Manufacture) discloses a method of manufacturing a steel pipe. This manufacture method proposes heating at a temperature in the range of 750-400° C., followed by rolling to a reduction rate in the range of 20% or more, or 60% or more, so as to obtain a steel pipe product having a yield strength of 950 MPa or more and good toughness. Nevertheless, due to the low heating temperature used by this process technique, rolling is difficult. In addition, the low rolling temperature tends to produce a martensite structure which is a microstructure not allowed to occur in an oil casing pipe product.

SUMMARY

An object of the disclosure is to provide an ultrahigh strength and ultrahigh toughness oil casing pipe which has both ultrahigh strength and ultrahigh toughness, wherein the strength can be up to 150 ksi steel grade or higher, and its zero degree lateral Charpy impact work is not less than 10% of the yield strength of the 150 ksi steel grade, thereby meeting the requirements of deep well and ultradeep well oil and gas fields for the strength and toughness of oil well pipes.

In order to fulfill the above object, the disclosure proposes an ultrahigh strength and ultrahigh toughness oil casing pipe, comprising the following chemical elements in mass percentages:

-   -   C: 0.12-0.18%;     -   Si: 0.1-0.4%;     -   Mn: 1.1-1.6%;     -   Cr: 0.1-0.4%;     -   Mo: 0.2-0.5%;     -   Nb: 0.02-0.04%;     -   Ti: 0.02-0.05%;     -   B: 0.0015-0.005%;     -   Al: 0.01-0.05%;     -   Ca: 0.0005-0.005%;     -   N≦0.008%;     -   with the proviso of 0<(Ti—3.4N)≦0.02%,     -   Ti/B≧10; and     -   the balance of Fe and other unavoidable impurities.

The unavailable impurities in this technical solution are mainly elements P and S, wherein the content of P is controlled to be ≦0.015%, and the content of S is controlled to be ≦0.003%.

The principle for designing the various chemical elements in the ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure is described as follows:

C: C is a carbide forming element, and it can increase steel strength. If the carbon content is lower than 0.12wt. %, steel hardenability will be decreased, leading to decreased steel toughness. However, if the carbon content is higher than 0.18 wt. %, segregation of steel will be exasperated remarkably, also leading to decreased steel toughness. In order to meet the high strength and toughness requirements of an oil casing pipe, the content of carbon element should be controlled in the range of 0.12-0.18wt. % according to the technical solution of the disclosure.

Si: Si is solid-dissolved in ferrite, and it can increase yield strength of steel. However, addition of an excessive amount of silicon element is undesirable, because excessive silicon element will degrade processability and toughness of steel. A silicon element content of lower than 0.1 wt. % may render an oil casing pipe susceptible to oxidation. Thus, the silicon content should be controlled in the range of 0.10-0.40 wt. %.

Mn: Mn is an austenite forming element, and it can increase steel hardenability. In a steel system for an ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure, if the manganese content is less than 1.1 wt. %, steel hardenability will be decreased significantly, leading to a decreased proportion of martensite in the steel, and further leading to decreased steel toughness. If the manganese content is more than 1.6 wt. %, structure segregation in the steel will be increased remarkably, thereby affecting homogeneity and impact performance of the hot rolled structure. For this reason, the manganese content is controlled in the range of 1.10-1.60 wt. % according to the technical solution of the disclosure.

Cr: Cr is an element that has a strong propensity to increase steel hardenability and form carbides. Chromium carbides precipitated during tempering can increase steel strength. Nonetheless, if the chromium content is higher than 0.4 wt. %, coarse carbide M₂₃C₆ tends to precipitate at grain boundaries, leading to decreased steel toughness; if the chromium content is lower than 0.1 wt. %, it will be hard to increase steel hardenability, indicating an insignificant effect of the addition of chromium. The chromium content is designed to be in the range of 0.1-0.4 wt. % in the ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure.

Mo: Mo increases steel strength and tempering stability mainly by carbide formation and solid-solution strengthening. In the technical solution of the disclosure, due to a low carbon content, if molybdenum is added in an amount of greater than 0.5 wt. %, it's hard for molybdenum to form more carbide precipitate phase with carbon, resulting in a waste of the added alloy. Once the molybdenum content is lower than 0.2 wt. %, the strength of the oil casing pipe cannot meet the requirement of high strength. For this reason, the molybdenum content is controlled in the range of 0.2-0.5 wt. % according to the disclosure.

Nb: Nb is an element for grain refinement strengthening and precipitation strengthening in steel, which can compensate the loss of strength caused by a decreased carbon content. If the niobium content is less than 0.02 wt. %, the effect of its addition is not significant; if the niobium content is more than 0.04 wt. %, coarse Nb (CN) tends to form, thereby decreasing steel toughness. Therefore, the niobium content in the technical solution of the disclosure should be controlled in the range of 0.02-0.04 wt. %.

Ti: Ti is an element that has a strong propensity to form carbides and nitrides. It can refine austenite grains in steel dramatically, and compensate the loss of strength caused by a decreased carbon content. If the titanium content is >0.05wt. %, coarse TiN tends to form, which will decrease the toughness of the material; if the titanium content is <0.02 wt. %, titanium cannot react fully with nitrogen to form TiN, in which case boron in the steel will react with nitrogen to form a brittle phase of BN, leading to decreased material toughness. The titanium content should be controlled in the range of 0.02-0.05 wt. % in the ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure.

B: B is also an element that can increase steel hardenability significantly. In steel having a low carbon content, boron element can solve the problem of poor hardenability caused by the low carbon content. However, if the boron content is lower than 0.0015 wt. %, the effect for improving steel hardenability is not obvious; if the boron content is higher than 0.005 wt. %, a brittle BN phase tends to form, thereby decreasing steel toughness. Therefore, the boron content in the technical solution of the disclosure is set to be 0.0015-0.005 wt. %.

Al: Al element is an excellent element for deoxygenation and azotification, and it can refine grains. It's desirable to adopt a content of 0.01-0.05% by weight percentage.

Ca: Ca is an element that can purify molten steel. It can promote balling of MnS, and increase impact toughness of a steel material. However, if the calcium content is unduly high, coarse non-metal inclusions tend to form in the steel. Therefore, the calcium content in the technical solution of the disclosure should be controlled in the range of 0.0005-0.005 wt. %.

N: The content of nitrogen element in the technical solution of the disclosure should be minimized.

Meanwhile, in order to guarantee sufficient bonding of titanium with nitrogen to avoid formation of a brittle BN phase from boron and nitrogen thereby decreasing the toughness index of a steel material, Ti, B and N among the above elements should further satisfy the following formula:

-   -   0<(Ti—3.4N)≦0.02%; and     -   Ti/B≧10.

Furthermore, the ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure further comprises vanadium element in a range of 0<V≦0.1 wt. %.

Vanadium element can refine grains in steel, and carbides formed with participation thereof can increase steel strength markedly. Nevertheless, when the amount of vanadium added is increased to a certain level, its further reinforcing effect is not obvious. Hence, for the technical solution of the disclosure, if added, the amount of vanadium element is ≦0.10 wt %.

Furthermore, the microstructure in the ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure is tempered sorbite.

To impart well matched strength and toughness to a casing pipe, the microstructure in the steel is tempered sorbite. This microstructure, formed by transformation of martensite, exhibits the best obdurability. The more martensite structure formed after hardening of a steel material, the more tempered sorbite structure will be obtained by transformation.

Since dendritic segregation in solidification of a steel pipe blank may result in a lot of segregation bands present in a rolled pipe body, alloy elements such as C, Mn, Cr, Mo and the like are enriched in the segregation bands, leading to non-uniform local distribution of the alloy components, so that a good number of coarse carbides are formed in the segregation bands. Meanwhile, the hardness and the strength of the steel in the segregation bands are rather high, leading to decreased toughness. For the purpose of reducing segregation of the components in the casing pipe, a measure may be taken to reduce C, Mn, Cr, Mo and other alloy elements. However, on the other hand, in order to provide the casing pipe with well matched strength and toughness, the microstructure of the steel material is tempered sorbite. This microstructure, formed by transformation of martensite, exhibits the best obdurability. The more martensite structure formed after hardening of a steel material, the more tempered sorbite structure will be obtained by transformation. As can thus be seen, improvement of hardenability to obtain more martensite structure is a key factor for ensuring obtainment of obdurability for the material. If a measure is taken to reduce C, Mn, Cr, Mo and other alloy elements to obtain a structure with low segregation so as to increase steel toughness, steel hardenability will be decreased inevitably, and thus steel obdurability will be decreased. Therefore, for high strength and high toughness steel, segregation in the steel and hardenability of the steel need to be properly balanced. According to the technical solution of the disclosure, a low carbon and low alloy composition system is used to obtain a microstructure with low segregation, and B and Ti are added at the same time to increase hardenability so as to improve steel toughness, thereby ensuring obtainment of uniform tempered sorbite.

Accordingly, the disclosure further proposes a method of manufacturing the above ultrahigh strength and ultrahigh toughness oil casing pipe, which method comprises the following steps: smelting; continuous casting; piercing; rolling; sizing; and heat treatment.

Further, in the above continuous casting step in the method of manufacturing the ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure, a molten steel superheat is controlled to be less than 30° C., and a continuous casting speed is controlled in the range of 1.8-2.2 m/min.

Control of the continuous casting speed in the range of 1.8-2.2 m/min serves to reduce segregation of the components in the steel.

Further, in the above piercing step in the method of manufacturing the ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure, a round blank from the continuous casting step is heated homogeneously in a furnace at 1200-1240° C., and a piercing temperature is 1180-1240° C.

Still further, in the above rolling step in the method of manufacturing the ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure, an end rolling temperature is controlled in the range of 900-950° C.

Still further, in the above sizing step in the method of manufacturing the ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure, a sizing temperature is 850-900° C.

Still further, in the above heat treatment step in the method of manufacturing the ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure, an austenitizing temperature is controlled in the range of 900-930° C., and the temperature is held for 30-60 min, followed by hardening; subsequently, tempering is conducted at a temperature in the range of 450-550° C., and this temperature is held for 50-80 min; finally, hot sizing is conducted at a temperature in the range of 400-550° C.

A low tempering temperature is used to impart the steel material with high strength. As a result, the cost for addition of alloys is reduced significantly while obdurability is increased.

The ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure can be used to manufacture an oil casing pipe of a steel grade of 150 ksi or higher that has an ultrahigh strength and an ultrahigh toughness.

A casing pipe of 150 ksi steel grade manufactured from the ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure has a yield strength of 1034-1241 MPa, a tensile strength ≧1103 MPa, an elongation of 20%-30%, a zero degree lateral Charpy impact work not less than 10% of the yield strength of 150 ksi steel grade (≧120 J), and a ductile-brittle transition temperature ≦−70° C.

A casing pipe of 155 ksi steel grade manufactured from the ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure has a yield strength of 1069-1276 MPa, a tensile strength ≧1138 MPa, an elongation of 20%-25%, a zero degree lateral Charpy impact work not less than 10% of the yield strength of 155 ksi steel grade (≧120 J), and a ductile-brittle transition temperature ≦−60° C.

For the ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure, owing to the addition of B, in place of Cr, Mo and other alloy elements added in a conventional steel, to increase steel hardenability, the oil casing pipe has reduced cost for alloy addition, and also has high strength and good toughness.

For the method of manufacturing the ultrahigh strength and ultrahigh toughness oil casing pipe according to the disclosure, the heat treatment process is controlled to impart high strength and good toughness to the steel material, wherein the process operation is simple, large-scale production can be realized easily, and good economical benefits can be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the metallographic structure of the ultrahigh strength and ultrahigh toughness oil casing pipe in Example A5.

FIG. 2 shows the morphology of the precipitated phase of the ultrahigh strength and ultrahigh toughness oil casing pipe in Example A5.

FIG. 3 shows the metallographic structure of the casing pipe in Comparative Example B 1.

FIG. 4 shows the morphology of the precipitated phase of the casing pipe in Comparative Example B2.

FIG. 5 shows the morphology of the precipitated phase of the casing pipe in Comparative Example B3.

DETAILED DESCRIPTION

The ultrahigh strength and ultrahigh toughness oil casing pipe of the disclosure and the method of manufacturing the same will be described in more detail with reference to the following specific Examples, but the specific Examples and the related description should not be construed to limit the technical solutions of the disclosure unduly.

Examples A1-A5 and Comparative Examples B1-B4

Casing pipes in Examples A1-A5 and Comparative Examples B1-B4 were manufactured according to the following steps:

1) Smelting: The mass percentages of the various chemical elements in Examples A1-A5 and Comparative Examples B1-B4 were controlled as shown in Table 1;

2) Continuous casting: Pipe blanks were made by continuous casting, wherein the molten steel superheat was controlled to be less than 30° C., and the continuous casting speed was controlled in the range of 1.8-2.2 m/min;

3) Piercing: the round blanks from the continuous casting step were heated homogeneously in an annular furnace at 1200-1240° C., and the piercing temperature was 1180-1240° C.;

4) Rolling: The end rolling temperature was controlled in the range of 900-950° C.;

5) Sizing: The sizing temperature was controlled in the range of 850-900° C.; and

6) Heat treatment: The austenitizing temperature was controlled in the range of 900-930° C., and the temperature was held for 30-60 min, followed by hardening; subsequently, tempering was conducted at a temperature in the range of 450-550° C., and this temperature was held for 50-80 min; finally, hot sizing was conducted at a temperature in the range of 400-550° C.

Table 1 lists the mass percentages of the various chemical elements in Examples A1-A5 and Comparative Examples B1-B4 in this disclosure.

TABLE 1 (wt %, the balance is Fe and other unavoidable impurities) No. C Si Mn Cr Mo Nb Ti B Al Ca N V Ti—3.4N Ti/B A1 0.12 0.2 1.1 0.1 0.2 0.03 0.02 0.0015 0.01 0.0005 0.004 — 0.0064 20 A2 0.13 0.1 1.2 0.2 0.3 0.02 0.025 0.002 0.04 0.001 0.005 0.03 0.008    12.5 A3 0.14 0.3 1.3 0.3 0.4 0.03 0.04 0.003 0.05 0.005 0.006 0.05 0.0196 13 A4 0.16 0.4 1.4 0.4 0.5 0.03 0.04 0.004 0.03 0.003 0.007 0.07 0.0162 10 A5 0.18 0.25 1.6 0.2 0.4 0.04 0.05 0.005 0.02 0.002 0.008 0.1 0.0128 10 B1 0.1  0.26 0.5 1   0.2 0.04 0.02 0.005 0.023 0.002 0.008 0.05 −0.0072   4 B2 0.15 0.33 1.2 0.5 0.3 0.03 — — 0.04 0.002 0.005 0.03 −0.017  B3 0.24 0.2 0.9 1   0.6 0.02 0.02 0.004 0.04 0.001 0.006 0.05 −0.0004   5 B4 0.18 0.3 1.2 0.3 0.4 0.04 0.02 0.004 0.05 0.003 0.008 0.06 −0.0072   5

Table 2 lists the various process parameters for the manufacture in Examples A1-A5 and Comparative Examples B1-B4 in this disclosure.

TABLE 2 Continuous Rolling casting step Piercing step step Sizing Heat treatment step Continuous Temperature End step Tempering Hot casting in Piercing rolling Sizing Austenitizing Holding tem- Holding sizing Superheat speed furnace temperature temperature temperature temperature time perature time temperature No. (° C.) (m/min) (° C.) (° C.) (° C.) (° C.) (° C.) (min) (° C.) (min) (° C.) A1 25 2 1220 1180 910 850 900 50 450 50 850 A2 10 2.2 1230 1210 900 860 930 30 480 60 860 A3 20 2.1 1240 1220 940 870 910 60 500 60 880 A4 30 1.8 1230 1190 950 880 920 60 520 80 890 A5 25 1.8 1200 1240 920 890 900 40 550 70 900 B1 20 1.9 1230 1210 920 900 900 40 500 70 870 B2 15 2.2 1240 1220 940 880 930 60 500 60 860 B3 20 1.9 1210 1230 950 870 910 40 500 60 890 B4 20 1.9 1220 1240 920 890 910 40 500 60 900

Table 3 lists the mechanical properties of the casing pipes involved in Examples A1-A5 and Comparative Examples B1-B4.

TABLE 3 Yield Tensile Lateral Ductile-brittle strength strength Elongation impact work, transition No. (MPa) (MPa) (%) 0° C. (J) temperature (° C.) A1 1050 1090 25 142 −80 A2 1070 1110 23 136 −70 A3 1090 1140 24 138 −70 A4 1120 1160 23 131 −60 A5 1100 1150 23 128 −60 B1 940 1010 23 125 −60 B2 960 1040 26 110 −55 B3 1090 1160 25 57 −25 B4 1070 1110 21 75 −30

As seen from Table 3, each of the casing pipes in Examples A1-A5 described above has a yield strength ≧1050 MPa (which is already above the strength of 150 ksi steel grade), a tensile strength ≧1090 MPa, a 0° lateral impact work ≧128 J, an elongation ≧23%, a ductile-brittle transition temperature ≦−60° C. That's to say, the casing pipes in Examples A1-A5 each have ultrahigh strength and ultrahigh toughness, suitable for manufacturing oil pipes for exploitation in deep and ultradeep wells. In contrast, as the Mn and Cr contents in Comparative Example B1 exceeded the ranges defined by the technical solution of the disclosure, no B or Ti was added in Comparative Example B2, the C, Mn, Cr and Mo contents in Comparative Example B3 exceeded the ranges defined by the technical solution of the disclosure, and the Ti and N contents in Comparative Example B4 failed to meet 0<(Ti—3.4N)≦0.02% and Ti/B≧10, the casing pipes in Comparative Examples B1-B4 have at least one mechanical property that does not reach the standard of an ultrahigh strength and ultrahigh toughness oil casing.

FIG. 1 shows the metallographic structure of the ultrahigh strength and ultrahigh toughness oil casing pipe in Example A5, and FIG. 2 shows the morphology of the precipitated phase of the ultrahigh strength and ultrahigh toughness oil casing pipe in Example A5.

As shown by FIG. 1, no banded structure resulting from composition segregation is found in the metallographic structure of the oil casing pipe in Example A5. As shown by FIG. 2, carbides in the precipitated phase of the oil casing pipe in Example A5 are fine and uniformly distributed. Hence, the ultrahigh strength and ultrahigh toughness oil casing pipe in Example A5 has a strength equal to or above 150 ksi steel grade and a zero degree lateral impact toughness of 120 J or higher.

FIG. 3 shows the metallographic structure of the casing pipe in Comparative Example B1.

Due to low C and Mn contents in Comparative Example B1, the hardenability of the steel is poor. As shown by FIG. 3, a considerable amount of ferrite structure is present in the metallographic structure of Comparative Example B1, so the strength of the heat treated casing pipe is insufficient, and its 0° lateral impact work is not high, rendering it unsuitable for manufacturing an ultrahigh strength and ultrahigh toughness oil casing pipe.

FIG. 4 shows the morphology of the precipitated phase of the casing pipe in Comparative Example B2, and FIG. 5 shows the morphology of the precipitated phase of the casing pipe in Comparative Example B3.

Since dendritic segregation during solidification of a steel pipe blank may result in a lot of segregation bands present in a rolled pipe body, as shown by FIG. 4, alloy elements such as C, Mn, Cr, Mo and the like were enriched in the segregation bands of Comparative Example B2, leading to non-uniform local distribution of the alloy components, so that a good number of coarse carbides were formed in the segregation bands.

As further shown by FIG. 5, the alloy element contents of C, Cr, Mo and the like in Comparative Example B3 exceeded the ranges defined by the technical solution of the disclosure, leading to serious segregation in the heat treated casing pipe, which in turn leads to insufficient toughness of the casing pipe and decreases the toughness index of the steel.

It is to be noted that there are listed above only specific Examples of the disclosure. Obviously, the disclosure is not limited to the above Examples. Instead, there exist many similar variations. All variations derived or envisioned directly from the disclosure of the disclosure by those skilled in the art should be all included in the protection scope of the disclosure. 

1. An ultrahigh strength and ultrahigh toughness oil casing pipe, comprising the following chemical elements in mass percentages: C: 0.12-0.18%; Si: 0.1-0.4%; Mn: 1.1-1.6%; Cr: 0.1-0.4%; Mo: 0.2-0.5%; Nb: 0.02-0.04%; Ti: 0.02-0.05%; B: 0.0015-0.005%; Al: 0.01-0.05%; Ca: 0.0005-0.005%; N≦0.008%; with the proviso of 0<(Ti—3.4N)≦0.02%, Ti/B≧10; and the balance of Fe and other unavoidable impurities.
 2. The ultrahigh strength and ultrahigh toughness oil casing pipe of claim 1, further comprising V: 0<V≦0.1 wt %.
 3. The ultrahigh strength and ultrahigh toughness oil casing pipe of claim 1, wherein the oil casing pipe has a microstructure of tempered sorbite.
 4. The ultrahigh strength and ultrahigh toughness oil casing pipe of claim 1, wherein the oil casing pipe has a yield strength of 1034-1241 MPa, a tensile strength ≧1103 MPa, an elongation of 20%-30%, a zero degree lateral Charpy impact work not less than 10% of the yield strength, and a ductile-brittle transition temperature ≦−70° C.
 5. The ultrahigh strength and ultrahigh toughness oil casing pipe of claim 1, wherein the oil casing pipe has a yield strength of 1069-1276 MPa, a tensile strength ≧1138 MPa, an elongation of 20%-25%, a zero degree lateral Charpy impact work not less than 10% of the yield strength, and a ductile-brittle transition temperature ≦−60° C.
 6. A method of manufacturing the ultrahigh strength and ultrahigh toughness oil casing pipe of claim 1, comprising the following steps: smelting; continuous casting; perforating; rolling; sizing; and heat treatment.
 7. The method of manufacturing the ultrahigh strength and ultrahigh toughness oil casing pipe according to claim 6, wherein, in the continuous casting step, a molten steel superheat is controlled to be less than 30° C., and a continuous casting speed is controlled in the range of 1.8-2.2 m/min.
 8. The method of manufacturing the ultrahigh strength and ultrahigh toughness oil casing pipe according to claim 6, wherein, in the piercing step, a round blank from the continuous casting step is heated homogeneously in a furnace at 1200-1240° C., and a piercing temperature is 1180-1240° C.
 9. The method of manufacturing the ultrahigh strength and ultrahigh toughness oil casing pipe according to claim 6, wherein, in the rolling step, an end rolling temperature is controlled in the range of 900-950° C.
 10. The method of manufacturing the ultrahigh strength and ultrahigh toughness oil casing pipe according to claim 6, wherein, in the sizing step, a sizing temperature is 850-900° C.
 11. The method of manufacturing the ultrahigh strength and ultrahigh toughness oil casing pipe according to claim 6, wherein, in the heat treatment step, an austenitizing temperature is controlled in the range of 900-930° C., and the austenitizing temperature is held for 30-60 min, followed by hardening; subsequently, tempering is conducted at a temperature in the range of 450-550° C., and this temperature is held for 50-80 min; finally, hot sizing is conducted at a temperature in the range of 400-550° C. 